Transfer molding of an epoxy resin composition is a suitable encapsulating method for low cost mass production of semiconductor devices such as ICs and LSIs. This method has been used for years, while its performance has been improved in respect of reliability through improvement of the epoxy resin and the phenol resin used as a curing agent. However, in the recent market trend requiring miniaturization, weight saving, and increased performance of electronic equipment, semiconductor devices have also become highly integrated year by year. This trend has increased requirements for the epoxy resin composition for semiconductor encapsulating. A semiconductor for vehicles particularly requires the guarantee of operation under a severe high temperature environment in the engine compartment or the vehicle body. Not only mounting characteristics such as solder resistant properties that have been conventionally required, but also reliability during high temperature storage (hereinafter referred to as “HTSL” (high temperature storage life)) and high temperature operation (hereinafter referred to as “HTOL” (high temperature operation life)) are required HTSL refers to life under high temperature storage without electric current for operation, whereas HTOL refers to life under high temperature while the device is operated by applying electric current.
Generally, a semiconductor element and a lead frame in a semiconductor device are connected using a gold wire. On the lead frame side, in particular the gold wire is joined with aluminum pad by heat and ultrasonic waves. In this instance, gold and aluminum produces an alloy, which is referred to as eutectic crystal or an intermetallic compound. The intermetallic compound grows during storage under high temperature conditions and produces cracks and corrosion, which cause a problem of a connection resistance increase. On the other hand when the device is operated by causing a current to pass through under high temperature conditions, the phenomenon taking place is one under an the environment in which a flow of electrons is produced due to passage of an actual current. The behavior is different from that exhibited during high temperature storage without a current flow. Particularly under the conditions of a high temperature and large current, transfer of intermetallic compounds by electrons (electric migration) induces problems such as transfer of the intermetallic compounds in the cathode, cracks and voids in the junction area, an increase in resistance, and conduction failure.
In order to improve the high temperature storage life (HTSL) under high temperature conditions, a method of decreasing the amount of impurities in the epoxy resin composition as much as possible, a method of adding an ion catcher, a method of using a resin substantially not containing a flame retardant such as bromine and antimony oxide, and the like, are becoming common techniques of maintaining the reliability during high temperature storage. Although these techniques can reduce ionic impurities and improve HTSL, the actual working state of the semiconductor elements, that is, the high temperature operating life (HTOL) can not necessarily be satisfied (for example, Japanese Patent Application Laid-open No. 2004-035781 (2-20 pages), Japanese Patent Application Laid-open No. 2000-230111 (2-6 pages)).
The present invention has been achieved to solve these problems in conventional technologies. Therefore an object of the present invention is to provide an epoxy resin composition for semiconductor encapsulating and a semiconductor device having improved high temperature operation life (HTOL), particularly excelling in reliability during operation under a high temperature environment of more than 150° C. that occurs in vehicles and the like.